As is well known and historically, the control for the gas turbine engine has typically adjusted fuel flow in attempting to optimize operation of the engine. In a typical installation the fuel control monitors a plurality of engine operating parameters and processes these signals to produce outputs that would be indicative of a desired engine operation while assuring that the engine avoids surging, overheating and rich and lean flame out. To achieve this goal the computer portion of the control manifests a control logic that represents the operation of the engine and continuously schedules fuel flow to reflect the setting of the power lever. In engines, particularly of the military variety, the control also, independently, monitors engine variables to schedule the variable geometry portions of the engine such as inlet guide vanes, exhaust nozzles and the like, to likewise attain optimum operation for any given operation within the engine's operating envelope.
Hence, it is apparent that a change in one control function would affect the condition of others so that there would be constant iterations of each of the controls to assure optimum operation of each. For example, a change in the exhaust nozzle area would typically change the pressure within the engine, which pressure would be monitored by the fuel control, which in turn would manifest a change in the fuel control to ultimately adjust fuel flow to reflect this change. In this process the scheduling of the fuel flow either to increase or decrease fuel will occur even prior to the time it takes the variable geometry of the engine to react. This "bootstrapping" effect has been addressed in U.S. application Ser. No. 246,765 entitled Active Geometry Control System for Gas Turbine Engines, supra.
In this co-pending application, supra, the control attains a faster thrust response and improved surge margin by synchronously scheduling fuel flow and the variable geometries of the engine in response to a single parameter which is a function of certain engine and aircraft operating variables. In order for this type of active geometry control system to be a useful system, it must be able to attain a high degree of repeatability in assuring that for any given setting the control will return to a given steady state operating point in the operating curve after any transition excursion. During a transient excursion, the control logic will assure that the point of operation is identical to the setting of the power lever which request is desired thrust even though the engine operating variable changes as a result of wear and tear of the engine, power extraction or compressor air bleed.
This invention contemplates utilizing a corrected airflow of the low pressure spool (Wa) of a twin spool engine as the primary control parameter. As is the case in many of the military engines, the low pressure spool is only aerodynamically coupled to the high pressure spool. In order to attain the optimum engine operation from a performance standpoint, the corrected rotor speeds for the high and low pressure spool must be proportional to each other for every given steady-state engine condition.
In fighters and other military aircraft, it is extremely important that a demand by the pilot for a change in thrust produced by the engine be as fast and as accurate as possible. The aircraft's ability to undergo the violent maneuvers anticipated when operating in the combat box, for example, in fighter aircraft, bears directly on the performance of that aircraft. When a demand for a thrust change is initiated, for example, when the pilot exercises a bodie maneuver, i.e., a quick demand for a drop in thrust (decel) followed by an immediate demand for an increase in thrust (accel) or vice versa, the engine should attain the demanded thrust levels by decelerating to the desired thrust level before accelerating to the desired thrust level in the quickest time possible. With heretofore known control logic, maneuvers, such as these bodies as well as chops, are influenced by the constraints owing to the high inertia of the rotating spool. Since a thrust change necessitates a decrease or increase in RPM of the low pressure compressor, this high inertia adversely affects the time responsiveness of the engine.
Needless to say, it is also extremely important that the engine operates as efficiently as possible to achieve good TSFC (thrust specific fuel consumption) and stable engine operating conditions, namely, avoiding surge, engine flame out and overheating.
It has been found that by controlling both the fuel and the fan variable vanes as a function of corrected low pressure compressor airflow, both steady state and transient operations are enhanced. In transients, this invention contemplates locking in a fixed corrected low compressor speed (N.sub.1), setting a target for the desired thrust and zeroing in on this target by adjusting the angle of the fan variable vanes. This logic allows the high pressure compressor spool to adjust speed to a value corresponding to the targeted airflow (Wa). A proportional plus integral controller assures that the engine airflow (Wa) is properly attained while N.sub.1 is held constant. Once the target is reached, the active low compressor controller (ALCC) automatically trims the low pressure compressor spool speed and hence the low compressor speed and FCVV to return to the operating line at a value that is equivalent to the desired speed ratio of the high and low pressure compressor to assure optimum engine performance.